Prevention and/or suppression of cancer was achieved in several different mouse systems. Passive antibodies (IgG), active killed and live vaccines in several repetitive experiments successfully prevented leukemia (P less than .001) in AKR mice and prevented 3MC induced sarcomas in C3H/f mice (P less than .006). Although the protection offered (IgG) is easily explained by long term suppression of AKR virus (Akv-1 and Akv-2 genes) and suppression of the AKR recombinant oncogene, the mechanism of suppression of sarcomas in C3H/f mice which are free of virus expression is still to be determined. It appears in both instances that high titered circulating specific antiviral antibodies are sufficient to prevent the oncogenic rearrangement of the cell membranes. We obtained evidence in the C3H/f system which suggests that IgG having high titers to both ecotropic and xenotropic viruses (and possibly a recombinant) was responsible for protection. A common tumor antigen shared by all known rat tumors including KiMSV(NP) sarcogene transformed cells provided a transplantation cell mediated rejection system in which the KiMSV(NP) rat tumor cell not only rejects itself but also spontaneous lymphoma, 4NQO induced sarcomas, DMH induced colon cancers and polyoma induced tumors of the rat. BIBLIOGRAPHIC REFERENCES: Gardner, M.B., Klement, V., Henderson, B.E., Estes, J.D., Dougherty, M., Casagrande, J. and Huebner, R.J.: Efforts to control type C virus expression in wild mice. In Chirigos, M.A. (ed.): Control of Neoplasia by Modulation of the Immune System. New York, Raven Press, 1977, pp. 391-407. Huebner, R.J., Gilden, R.V., Lane, W.T., Trimmer, R.W. and Hill, P.R.: Suppression of endogenous type C RNA virogene expression in mice by serotype-specific viral vaccines; progress report. In Chirigos, M.A. (Ed.): Control of Neoplasia by Modulation of the Immune System. New York, Raven Press, 1977, pp. 381-389.